Project Summary/Abstract Pancreatic adenocarcinoma (PDA) is characterized by near universal mutation of the KRAS oncogene. Despite this common alteration, PDA tumors and cell lines can be classified into subtypes based on the expression of distinct cellular and molecular signatures. Importantly, this classification can predict clinical outcome, with the most aggressive subtype displaying features of a mesenchymal cell state. Taking advantage of this newly discovered heterogeneity in PDA, my ultimate goal is to define the mechanisms that distinguish these subtypes and the master regulators that drive subtype specification. My preliminary data have identified a key regulator of the aggressive subtype of PDA that can induce and maintain a poorly differentiated mesenchymal state. Moreover, PDA cells display remarkable cellular plasticity as they can dynamically switch between subtypes in response to specific stimuli, for instance, following silencing of mutant Kras. Motivated by this preliminary data, my specific aims will address two fundamental questions regarding subtype specification and heterogeneity in PDA: (1) What are the mechanisms that drive subtype specification? (2) How does subtype switching enable resistance to inhibition of key oncogenic drivers? This work will define the role of master regulators in maintaining specific PDA subtypes and broaden our understanding of the distinct molecular features associated with tumor heterogeneity. Finally, these studies have the potential to inform novel strategies for combating therapy resistance and tumor relapse.